Self-compensating mark design for stepper alignment

ABSTRACT

A system and method for fabricating integrated circuits using four fine alignment targets per stepper shot. The four alignment targets are disposed within the scribe line on each side of a four-sided stepper shot. The targets on opposites sides of the region are located in mirror-image positions. For example, in a square or rectangular region, the targets could be at the mid-point of each side, or at each corner. Because the scribe lines for adjoining stepper shots overlap, a target in one shot will overlay a target from a preceding shot. In a positive resist process, for example, the target resulting from the overlay will be reduced in size by an amount corresponding to the amount of rotational error, if any. However, the target will still indicate the center of the stepper shot, thereby compensating for the rotational error with no further measurements.

TECHNICAL FIELD

The present invention relates to the fabrication of integrated circuitson wafers in a stepper. More specifically, the present inventionpertains to an arrangement of fine alignment targets used for aligning astepper shot and a wafer.

BACKGROUND ART

Integrated circuit dies are fabricated en masse on silicon wafers usingwell-known techniques such as photolithography. Using these techniques,a pattern that defines the size and shape of the components andinterconnects within a given layer of the integrated circuit die isapplied to the wafer. The pattern applied to the wafer is laid out in anarray, or matrix, of reticle images. A wafer stepper holds the patternover a wafer and projects the pattern image of the reticle onto thewafer through a lens. The area on the wafer upon which the image isprojected is defined as a stepper shot.

Referring now to prior art FIG. 1A, a side view of a stepper 100 isshown. Stepper 100 includes a light source 122, masking blades 124, areticle 126, a lens 128, and a stage 112. The light source 122 projectslight through an opening 126 a of masking blades 124, through thetransparent portion of a pattern 126 a on reticle 126, through lens 128and onto a wafer 133 located on the stage 112. By doing so, the pattern126 a of the reticle 126 is reproduced on the wafer 133, typically at a5:1 reduction. A pattern located on an inner, or center, portion of thereticle 126 passes through a center portion 128 a of lens 128.Similarly, a pattern 126 b located on an outer, or peripheral, portionof the reticle 126 passes through an outer portion 128 b of lens 128.

The integrated circuit is essentially built-up by forming on the wafer133 a multitude of interconnecting layers, one layer on top of another.Because the layers interconnect, a need arises for ensuring that thepatterns on wafer 133 are accurately positioned and formed. Conventionalmethods rely on precise alignment of the wafer 133, the stage 112, thelens 128 and the reticle 126 in order to accurately fabricate anintegrated circuit.

Accurate formation of an image on a wafer using photolithography can beaffected by several error-causing variables. These variables includerotational alignment error, translational alignment error, and lensdistortion error, among others. Each one of these error-causingvariables can be corrected by a different part of the stepper. It isdesirable to segregate the types of errors and measure themindependently so that the error measurements are not confounded and sothat the resulting corrections for each variable will not be conflictingand counterproductive.

The rotational alignment error, caused by rotational movement of thereticle 126 relative to the wafer 133 (or vice versa), is of particularinterest with regard to the discussion herein. As described above, it isdesirable to segregate rotational error from the other error-causingvariables in order to compensate for the true measurement of rotationalerror.

With reference now to Prior Art FIG. 2A, alignment targets 14 and 16 areplaced on the wafer 133 in order to ensure that the final alignment ofthe wafer 133 and the reticle 126 (FIG. 1) is correct before theintegrated circuit is formed. The alignment targets 14 and 16 arelocated within the scribe line region of a stepper shot 12. A steppershot 12 may be comprised of multiple integrated circuit dies or a singledie. Multiple stepper shots are performed until the entire wafer 133 isexposed.

Prior Art FIG. 2A illustrates an arrangement using only two alignmenttargets 14 and 16. Alignment target 14 is used for acquiring they-direction offset, and alignment target 16 is used for acquiring thex-direction offset. In the prior art, a deviation of the alignmenttargets 14 and 16 from their expected location or orientation (asdefined in the controlling software) is interpreted as a translationalerror when, in actuality, it may be a rotational error or a lensdistortion error.

In order to perform a measurement of rotational error in the prior art,an additional alignment target 18 is required as shown by Prior Art FIG.2B. Prior art design guides specify that alignment target 18 must beplaced in a location where it will not interfere with the y-directionmeasurement mark (e.g., alignment target 14) on the y-coordinate.Consequently, alignment target 14 and alignment target 18 are notaligned with each other and are separated in the y-direction by anoffset 20. The amount of offset 20 is measured to determine the amountof rotational error. That is, for example, the amount of offset 20 isknown for the case where there is no rotational error. If stepper shot12 is rotated clockwise, the amount of offset 20 will increase relativeto this amount, and the amount of the increase can be translated to ameasurement of rotational error.

Thus, to determine the amount of rotational error in the prior art, allthree alignment targets 14, 16 and 18 need to be acquired and thedeviation from their expected positional values measured. In somestepper implementations, the targets are acquired for multiple shots inorder to obtain the measurements needed to determine rotational error.Accordingly, the alignment targets 14, 16 and 18 may need to be acquiredand measured multiple times per wafer. The time and the processingeffort needed to acquire the targets, obtain measurements, and calculaterotational error can limit the throughput of the stepper.

Furthermore, proper focusing of the alignment scope used to acquiretargets 14, 16 and 18 is required in order to acquire the targets withthe precision necessary for calculating rotational error. This focusingmay be performed for every wafer or at some other frequency (e.g., everyother wafer, every fifth wafer, etc.). At any rate, the time needed tocomplete the focusing task can further limit the throughput of thestepper.

Accordingly, what is needed is a method and/or system that can properlycompensate for rotational error in the integrated circuit fabricationprocess. What is also needed is a method and/or system that can satisfythe above need and that can save measurement and processing time,thereby potentially improving stepper throughput. The present inventionprovides a novel solution to the above needs.

DISCLOSURE OF THE INVENTION

The present invention provides a method and system thereof that canproperly compensate for rotational error in the integrated circuitfabrication process. The present invention also provides a method andsystem that can satisfy the above need and that can save measurement andprocessing time, thereby potentially improving stepper throughput.

The present embodiment of the present invention pertains to a method andsystem thereof for fabricating integrated circuits using four finealignment targets per stepper shot. In one embodiment, the presentinvention pertains to a wafer that has four fine alignment targets perstepper shot. In another embodiment, the present invention pertains to areticle used to form the four alignment targets on the wafer.

The four alignment targets are formed within the scribe line on eachside of a four-sided stepper shot. In accordance with the presentinvention, the alignment targets on opposites sides of the stepper shotare located in mirror-image positions. In one embodiment, for a squareor rectangular stepper shot, the alignment targets are located at themid-point of each side of the stepper shot. In another embodiment, thealignment targets are located at each corner of the stepper shot.

In one embodiment, the alignment targets each include a plurality ofrectangles that have prescribed dimensions (e.g., length and width). Inone embodiment, the alignment targets are formed using a positive resistprocess and thus will consist of solid (e.g., chrome) rectangles in aclear field. In another embodiment, the alignment targets are formedusing a negative resist process and thus will consist of a dark field(e.g., chrome) background with rectangular windows (e.g., non-chromeregions) formed therein.

Because the scribe lines for adjoining stepper shots overlap, a secondalignment target formed in one shot will overlay a first alignmenttarget formed in a preceding shot (e.g., a right-hand target in a firststepper shot will be overlaid by the left-hand target in the adjoiningstepper shot). With no rotational error (no rotation of the stepper shotrelative to the wafer), the first and second targets will be aligned,and the rectangles that make up the alignment target will retain theirprescribed dimensions.

On the other hand, with the stepper shot rotated relative to the wafer,the first and second alignment targets in adjoining stepper shots willnot be aligned, and the dimensions of the rectangles that make uptargets will change by an amount corresponding to the amount ofrotation. That is, with solid rectangles in a clear field, the width ofthe rectangles will be reduced by an amount corresponding to the amountof rotational error, and with clear rectangles in a dark field, thewidth of the rectangles will be increased by an amount corresponding tothe amount of rotational error.

Nevertheless, in accordance with the present invention, for the case inwhich the stepper shot is rotated about its center relative to the wafer(or vice versa), the center line of the rectangles, and hence thealignment target, will remain unchanged even with the rotation, therebycompensating for the rotational error without the need for measurement.Thus, the effect of the rotational error is, in essence, canceled out bythe placement of the four alignment targets in accordance with thepresent invention.

For the case in which the stepper shot is rotated about the center ofone of its sides (e.g., about the left-center of the shot), therotational error is reduced by one-half. Thus, in accordance with thepresent invention, the rotational error for this latter case isminimized and will therefore consume a smaller portion of the budgetoverlay requirement.

Thus, in accordance with the present invention, the alignment targetresulting from the overlay of alignment targets in adjoining steppershots will compensate for rotation about the center of a stepper shot.Consequently, it is not necessary to acquire the alignment targets inorder to determine rotational error, saving time and reducing processingand thereby potentially increasing stepper throughput. In addition,because the alignment approach of the present invention uses fouralignment targets per stepper shot, the precision of the fabricationprocess can be increased relative to conventional approaches.

These and other objects and advantages of the present invention willbecome obvious to those of ordinary skill in the art after having readthe following detailed description of the preferred embodiments whichare illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

PRIOR ART FIG. 1 is a side view of one embodiment of a stepper inaccordance with the present invention.

PRIOR ART FIGS. 2A and 2B show arrangements of alignment targetsaccording to the prior art.

FIG. 3 is a block diagram of one embodiment of a stepper in accordancewith the present invention.

FIG. 4A shows the general layout of a integrated circuit die inaccordance with one embodiment of the present invention.

FIG. 4B shows an arrangement of alignment targets according to oneembodiment of the present invention.

FIG. 4C shows an arrangement of alignment targets according to anotherembodiment of the present invention.

FIG. 5 shows target patterns in an alignment target in accordance withone embodiment of the present invention.

FIG. 6A illustrates a target mark in an alignment target with norotational error in accordance with one embodiment of the presentinvention.

FIG. 6B illustrates a target mark in an alignment target with clockwiserotation about the center of a stepper shot in accordance with oneembodiment of the present invention.

FIG. 6C illustrates a target mark in an alignment target with clockwiserotation about the left-center of a stepper shot in accordance with oneembodiment of the present invention.

FIG. 7 is a flowchart of the steps in a process for forming alignmenttargets on a wafer in accordance with one embodiment of the presentinvention.

FIG. 8 illustrates a top perspective view of a wafer with alignmenttargets in accordance with one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail so as not to unnecessarily obscure aspects of the presentinvention.

Some portions of the detailed descriptions which follow are presented interms of procedures, logic blocks, processing, and other symbolicrepresentations of operations for fabricating integrated circuits on awafer. These descriptions and representations are the means used bythose skilled in the art of wafer fabrication to most effectively conveythe substance of their work to others skilled in the art. In the presentapplication, a procedure, logic block, process, or the like, isconceived to be a self-consistent sequence of steps or instructionsleading to a desired result. The steps are those requiring physicalmanipulations of physical quantities. Usually, although not necessarily,these quantities take the form of electrical or magnetic signals capableof being stored, transferred, combined, compared, and otherwisemanipulated in a computer system to fabricate an integrated circuit.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present invention,discussions utilizing terms such as “receiving,” “performing,”“forming,” “overlaying,” or the like, refer to actions and processes(e.g., process 700 of FIG. 7) of integrated circuit fabrication.

Refer now to FIG. 3, which shows a block diagram of a stepper 202 inaccordance with one embodiment of the present invention. Stepper 202includes a stage 208 coupled to a stage movement device 210, a processor212, a signal transceiver 204, and a memory 214. Wafer 206 is placed onstage 208 for processing in stepper 202.

Memory 214 contains program instructions implemented through processor212. Memory 214 can either be permanent memory, such as read only memory(ROM), or temporary memory, such as random access memory (RAM). Memory214 can also be any other type of memory storage capable of containingprogram instructions, such as a hard drive, a CD ROM, or flash memory.Processor 212 can either be an existing system processor ormicroprocessor, a dedicated digital signal processing (DSP) processorunit, or a dedicated controller or microcontroller. Alternatively, theprogram instructions may be implemented using an implementation of astate machine.

Signal transceiver 204 is coupled to processor 212. Signal transceiver204 is a source of an electromagnetic signal, such as a laser.Additionally, signal transceiver 204 is a receiver for anelectromagnetic signal, such as the return signal that is reflected fromwafer 206.

FIG. 4A shows the general layout of a single integrated circuit dieelement 400 on a wafer (e.g., wafer 206 of FIG. 3). Although die element400 is illustrated as square-shaped, it is appreciated that die element400 may be rectangular-shaped. Die element 400 comprises integratedcircuit die 401, which is surrounded by an input/output (I/O) pad area402, a guard ring 403, and a scribe line 404. The scribe line 404contains inserts and production marks (e.g., the alignment marks of FIG.4B or 4C) used during the manufacture of integrated circuit dies from awafer. When die element 400 is stepped around the wafer 206 duringmanufacture, the scribe line 404 is overlaid from side to side and fromtop to bottom. That is, a segment of the scribe line in one stepper shotis overlaid by a segment of the scribe line of the adjoining steppershot. For example, the right-hand portion of a scribe line of a firststepper shot is overlaid by the left-hand portion of a scribe line ofthe adjoining stepper shot.

FIG. 4B shows an arrangement of alignment targets 420 a, 420 b, 430 aand 430 b in a stepper shot 410 on a wafer 206 according to oneembodiment of the present invention. Although stepper shot 410 isillustrated as square-shaped, it is appreciated that stepper shot 410may be rectangular-shaped. Alignment targets 420 a, 420 b, 430 a and 430b are disposed within the scribe line of stepper shot 410 (e.g., scribeline 404 of FIG. 4A). It will be understood by a person of ordinaryskill in the art that the arrangement of alignment targets illustratedby FIG. 4B also corresponds to a target pattern on a reticle used toform alignment targets 420 a, 420 b, 430 a and 430 b on wafer 206 instepper shot 410.

With reference to FIG. 4B, alignment targets 420 a and 420 b are onopposite sides of stepper shot 410 and are positioned in mirror-imagelocations. In the present embodiment, alignment targets 420 a and 420 bare located at the mid-point of opposite sides of stepper shot 410.Similarly, alignment targets 430 a and 430 b are positioned inmirror-image locations on the other two opposite sides of stepper shot410. In the present embodiment, alignment targets 430 a and 430 b arelocated at the mid-point of opposite sides of stepper shot 410. It isappreciated that, in other embodiments, alignment targets 420 a, 420 b,430 a and 430 b can be positioned at other than the mid-points of thesides of stepper shot 410, as long as alignment targets 420 a and 420 bare in mirror-image positions on opposite sides, and alignment targets430 a and 430 b are in mirror-image positions on the other two oppositesides.

FIG. 4C shows an arrangement of alignment targets 440 a, 440 b, 440 cand 440 d according to another embodiment of the present invention. Inthis embodiment, alignment targets 440 a, 440 b, 440 c and 440 d arelocated at the corners of stepper shot 410.

With reference to FIGS. 4A, 4B and 4C, when die element 400 is steppedaround wafer 206, the scribe lines 404 for each stepper shot 410 areoverlaid by an adjoining stepper shot, as described above. Accordingly,the alignment targets in the scribe lines 404 will also be overlaid byan alignment target of an adjoining stepper shot. For example, alignmenttarget 430 b in a first stepper shot will be overlaid by alignmenttarget 430 a in a subsequent stepper shot adjoining, and to the rightof, the first stepper shot.

As will be seen, the use of four alignment targets, and theirarrangement in mirror-image positions in the scribe line of a steppershot, provides an efficient and precise method and system forsegregating and compensating for rotational error (refer to FIGS. 6Athrough 6C, below). In addition, the use of four alignment targets perstepper shot will increase the precision of the alignment process.Furthermore, with four alignment targets arranged as described for thepresent invention, lens distortion error may be averaged out or reduced.

FIG. 5 shows target patterns 500 a and 500 b in an alignment target(e.g., the targets of FIGS. 4B and 4C) in accordance with one embodimentof the present invention. The target pattern 500 a comprises a pluralityof solid (typically, chrome) rectangles 510 in a clear field 512. Thetarget pattern 500 b comprises a plurality of clear rectangles 520 in asolid (typically, chrome) background 522. The target patterns 500 a and500 b are formed using a known positive or negative photoresist process.

Although six rectangles are shown in each of target patterns 500 a and500 b, it is understood that any number of such rectangles can be usedin accordance with the present invention. Furthermore, other types oftarget patterns and designs can be used in accordance with the presentinvention; one such design is described in U.S. Pat. No. 5,316,984 by P.Leroux and assigned to the assignee of the present invention, and whichis hereby incorporated by reference. It is also understood that thetarget patterns of FIG. 5 correspond to target patterns on a reticlethat is used to form target patterns 500 a and 500 b on wafer 206 instepper shot 410 (FIGS. 4B and 4C).

Continuing with reference to FIG. 5, the dimensions of rectangles 510and 520 are precisely specified. Typically, the rectangles 510 and 520have a width of four (4) microns and a length of 30 microns. However,these dimensions are not critical to the present invention, anddifferent dimensions can be used.

In accordance with one embodiment of the present invention, FIG. 6Aillustrates the alignment of a target mark (e.g., a rectangle 510) in analignment target (e.g., the alignment targets of FIGS. 4B and 4C) forthe case where there is no rotational error. Recall that, in accordancewith the present invention, the alignment target of one stepper shotwill be overlaid by the alignment target of a subsequent, adjoiningstepper shot; for example, alignment target 430 b in a first steppershot will be overlaid by alignment target 430 a in a subsequent steppershot adjoining, and to the right of, the first stepper shot. For thepurposes of the discussion herein, the alignment target formed as aresult of the overlay of alignment targets will be referred to as the“overlay target” or the “resultant target.”

For the case in which there is no rotational error, the target marks(e.g., rectangles 510) of alignment target 430 a (from the secondstepper shot) and the target marks of alignment target 430 b (from thefirst stepper shot) will precisely align. As a result, there will be nochange in the specified dimensions of each rectangle 510 in the overlaytarget (e.g., each rectangle 510 will still measure 4 microns by 30microns). Consequently, the center (centroid) of each rectangle 510 inthe overlay target is unchanged, and the centroid of the overlay targetis also unchanged.

Thus, in accordance with the present invention, when the dimensions ofthe rectangles 510 in the overlay target are unchanged, this provides anindication that there is no rotational error. Conversely, when alignmenttarget 430 b (from the first stepper shot) and alignment target 430 a(from the second stepper shot) align, this also provides an indicationthat there is no rotational error. As such, measurements andcalculations needed to determine rotational error are not necessary,saving both fabrication time and processing time and potentiallyincreasing the throughput of the stepper.

FIG. 6B illustrates a target mark (e.g., rectangle 510) in an alignmenttarget (e.g., the alignment targets of FIGS. 4B and 4C) for the case inwhich there is clockwise rotation about the center of each stepper shot,in accordance with one embodiment of the present invention. FIG. 6B canalso illustrate the case in which a first stepper shot is properlyaligned (no rotational error) and a second, adjoining stepper shot isrotated about its center. As a result in either case, the alignmenttarget 430 a of the second stepper shot will not precisely align withthe alignment target 430 b of the first stepper shot. Consequently, atarget mark (e.g., rectangle 630) of alignment target 430 a (of thesecond stepper shot) will not align with a target mark (e.g., rectangle632) of alignment target 430 b (of the first stepper shot).

According to the positive photoresist process, the rectangle 510resulting from the overlay of rectangles 630 and 632 will be formed.Accordingly, rectangle 510 of the overlay target will have reduceddimensions (that is, its width will be something less than 4 microns,depending on the amount of rotation). However, for the case in which therotation occurs about the center of a stepper shot, the center(centroid) of rectangle 510 will not change, and thus the centroid ofthe overlay target is also unchanged. Note that, for the negative resistprocess, the rectangle 510 resulting from the overlay of rectangles 630and 632 will have increased dimensions (that is, its width will besomething more than 4 microns, depending on the amount of rotation).

Therefore, in accordance with the present invention, when there isrotation about the center of a stepper shot, the effect of the rotationis canceled out because the centroid of the overlay target is unchanged.That is, the centroid of each resultant rectangle 510 in the overlaytarget, and hence the centroid of the overlay target, remains coincidentwith the center of the stepper shot. Accordingly, the rotation of thestepper shot can be compensated for by finding the centroid of theoverlay target, as if there was no rotation. As such, measurements andcalculations needed to determine rotational error are not necessary,saving both fabrication time and processing time and potentiallyincreasing the throughput of the stepper.

FIG. 6C illustrates a target mark (e.g., rectangle 510) in an alignmenttarget (e.g., the alignment targets of FIGS. 4B and 4C) for the case inwhich there is clockwise rotation about the center of one side of eachstepper shot, in accordance with one embodiment of the presentinvention. FIG. 6C can illustrate the case in which, for example, afirst stepper shot is properly aligned (no rotational error) and asecond, adjoining stepper shot is rotated about its left-center. As aresult in either case, the alignment target 430 a of the second steppershot will not precisely align with the alignment target 430 b of thefirst stepper shot. Consequently, a target mark (e.g., rectangle 640) ofalignment target 430 a (of the second stepper shot) will not align witha target mark (e.g., rectangle 642) of alignment target 430 b (of thefirst stepper shot).

According to the positive photoresist process, the rectangle 510resulting from the overlay of rectangles 640 and 642 will be formed.Accordingly, rectangle 510 of the overlay target will have reduceddimensions (that is, its width will be something less than 4 microns,depending on the amount of rotation). Note that, for the negative resistprocess, the rectangle 510 resulting from the overlay of rectangles 630and 632 will have increased dimensions (that is, its width will besomething more than 4 microns, depending on the amount of rotation).

In this case, where a stepper shot is rotated about the center of one ofits sides, the centroid of each resultant rectangle 510 in the overlaytarget will be different from the center of the stepper shot, and hencethe centroid of the overlay target will be different from the center ofthe stepper shot. Thus, there may be a need to measure and compensatefor the rotational error for this type of rotation. However, in general,rotation about the center of one side of the stepper shot does not occuras frequently as rotation about the center of the stepper shot.Moreover, in accordance with the present invention, the magnitude of therotational error is reduced by one-half. Accordingly, the rotationalerror of this type may not consume a significant portion of the overlaybudget. As such, in a greater number of instances relative to the priorart, it may be permissible to neglect the rotational error, and thusstepper performance will not be adversely affected and can bepotentially improved.

Therefore, in accordance with the present invention, when there isrotation about the center of a stepper shot, the effect of the rotationis canceled out because the centroid of the overlay target is unchanged.That is, the centroid of each resultant rectangle 510, and hence thecentroid of the overlay target, remains coincident with the center ofthe stepper shot. As such, measurements and calculations needed todetermine rotational error are not necessary, saving both fabricationtime and processing time and potentially increasing the throughput ofthe stepper. In other instances, when rotation occurs about the centerof one side of a stepper shot, the magnitude of the rotational error isreduced and therefore may not be a significant portion of the overlaybudget requirement.

FIG. 7 is a flowchart of the steps in a process 700 for formingalignment targets (e.g., the alignment targets of FIGS. 4B and 4C) on awafer 206 (FIGS. 4B and 4C) in accordance with one embodiment of thepresent invention. Many of the instructions for the steps, and the datainput and output from the steps, of process 700 utilize memory andutilize controller hardware shown in FIG. 3. For example, wafer 206,stage 208, and signal transceiver 204, can be controlled by memory 214and processor 212 to accomplish the requirements steps in process 700.Alternative embodiments of FIG. 3 are equally applicable to implementingthe steps of process 700. Furthermore, while in the present embodimentprocess 700 is utilized in a stepper machine, the present invention iswell-suited to other devices needing wafer alignment. In the presentembodiment, software in memory 214 does not need to be modified forprocess 700.

While process 700 of the present embodiment shows a specific sequenceand quantity of steps, the present invention is suitable to alternativeembodiments. For example, the present invention is well-suited to anembodiment which includes more or less steps than process 700. Likewise,the sequence of the steps can be modified depending upon theapplication. Furthermore, while process 700 is shown as a single serialprocess, it can also be implemented as a continuous or parallel process.

In step 710 of FIG. 7, a wafer 206 is received at stepper 202 (FIG. 3).

In step 720 of FIG. 7, with reference also to FIGS. 4A, 4B and 4C, afirst stepper shot 410 is performed. In accordance with the presentinvention, four alignment targets are formed in scribe line 404 ofstepper shot 410. In one embodiment, the four alignment targets arepositioned as shown by FIG. 4B; that is, alignment targets 420 a and 420b are located in mirror-image positions on opposite sides of steppershot 410, and alignment targets 430 a and 430 b are similarly located.In another embodiment, the four alignment targets 440 a-b are positionedas shown in FIG. 4C.

In step 730 of FIG. 7, with reference as well to FIGS. 4A, 4B and 4C, asecond stepper shot 410 with four alignment targets is performed. Asegment of scribe line 404 of the second stepper shot overlays a segmentof scribe line 404 of the first stepper shot, such that an alignmenttarget of the second stepper shot overlays an alignment target of thefirst stepper shot. For example, the left-hand portion of scribe line404 of the second stepper shot can overlap the right-hand portion ofscribe line 404 of the first stepper shot, in which case alignmenttarget 430 a of the second stepper shot would overlay alignment target430 b of the first stepper shot.

In accordance with the present invention, if there is no rotation of thestepper shots, then alignment target 430 b (of the second stepper shot)would precisely align with alignment target 430 a (of the first steppershot).

If there is rotation of one or both of the stepper shots about theircenters, then alignment target 430 b (of the second stepper shot) willnot precisely align with alignment target 430 a (of the first steppershot); however, the centroid of the overlay target formed as a result ofthe overlay will still indicate the center of the shot. Accordingly,there is not a need to measure or calculate the magnitude of therotational error, and the rotation of the second stepper shot can becompensated for by finding the centroid of the overlay target.

If there is rotation of one or both of the stepper shots about thecenters of one of their sides, then alignment target 430 b (of thesecond stepper shot) will not precisely align with alignment target 430a (of the first stepper shot). In this case, the centroid of the overlaytarget formed as a result of the overlay will be different from thecenter of the shot due to rotational error; however, the magnitude ofthe rotational error will be reduced by one-half relative toconventional methods.

FIG. 8 illustrates a top perspective view of a wafer 830 with amultiplicity of alignment targets 835 in accordance with one embodimentof the present invention. Wafer 830 is partitioned into a number ofshots exemplified by shot 840. In FIG. 8, for illustration purposes,shots 850, 852, 860, 862, 870, 872, 880 and 882 have four alignmenttargets 835 located at the mid-point of each their sides, as describedin conjunction with FIG. 4B, above. However, it is appreciated that eachshot 840 can have four alignment targets 835 in accordance with thepresent invention.

With reference still to FIG. 8, considering only adjacent shots 850 and852 by way of example, alignment target 836 is located at the right-handside of shot 850 and the left-hand side of shot 852. As described above,alignment target 836 is formed by overlapping the left-hand target ofshot 852 and the right-hand target of shot 850. Thus, alignment target836 is actually the composite of targets formed in two different shots.

In general, when aligning the stepper and the wafer, a certain number ofshots are selected for the alignment. Typically, eight shots may beselected for alignment. However, in accordance with the presentinvention, adjacent shots share alignment targets, as describedpreviously herein. Correspondingly, a target such as alignment target836 in effect represents two shots (e.g., shots 850 and 852). As such,in accordance with the present invention, the number of shots selectedfor alignment can be reduced by one-half while still maintaining thecurrent level of precision. On the other hand, the precision can beincreased two-fold if the number of shots selected for alignment is notreduced. Thus, another advantage to the present invention is that it canadvantageously increase throughput without loss of precision, orincrease precision without loss of throughput.

The present invention thus provides a method and system thereof that canproperly compensate for rotational error in the integrated circuitfabrication process. The present invention also provides a method andsystem that can save measurement and processing time, therebypotentially improving stepper throughput.

The preferred embodiment of the present invention, a self-compensatingmark design for stepper alignment, is thus described. While the presentinvention has been described in particular embodiments, it should beappreciated that the present invention should not be construed aslimited by such embodiments, but rather construed according to thefollowing claims.

1-7. (canceled)
 8. A reticle used to form alignment targets on a wafer,said reticle comprising: a four-sided region comprising a first patternfor forming an integrated circuit on said wafer, said region having ascribe line along its perimeter; and a second pattern disposed withinsaid scribe line and comprising four alignment target patterns forforming four alignment targets on said wafer in a stepper shot; whereinone alignment target pattern is located on each side of said region, andwherein an alignment target pattern on a first side of said region andan alignment target pattern on a second side of said region opposingsaid first side are located in mirror image positions.
 10. The reticleas recited in claim 8 wherein opposing sides of said region are equal inlength, and wherein each of said alignment target patterns is located ata mid-point of a side of said region.
 11. The reticle as recited inclaim 8 wherein each of said four alignment target patterns is locatedat a corner of said region.
 12. The reticle as recited in claim 8wherein said alignment target patterns comprise a plurality ofrectangular-shaped masks.
 13. The wafer as recited in claim 8 whereinsaid alignment targets are formed according to a positive resistprocess.
 14. The wafer as recited in claim 8 wherein said alignmenttargets are formed according to a negative resist process.
 15. In astepper, a method for forming alignment targets on a wafer, said methodcomprising the steps of: a) receiving said wafer; b) performing a firststepper shot according to a pattern mask covering a four-sided regionhaving a scribe line along its perimeter; c) forming four alignmenttargets within said scribe line of said first stepper shot, wherein onealignment target is disposed on each side of said first stepper shot andwherein an alignment target on a first side of said first stepper shotand an alignment target on a second side opposing said first side arelocated in mirror-image positions; d) performing a second stepper shotadjoining said first stepper shot and using said pattern mask of saidstep b), wherein a segment of a scribe line of said second stepper shotoverlays a segment of said scribe line of said first stepper shot; ande) forming four alignment targets within said scribe line of said secondstepper shot such that an alignment target of said second stepper shotoverlays an alignment target of said first stepper shot.
 16. The methodas recited in claim 15 wherein opposing sides of said first and secondstepper shots are equal in length, and wherein an alignment target islocated at each mid-point of a side of said stepper shots.
 17. Themethod as recited in claim 15 wherein an alignment target is located ateach corner of said first and second stepper shots.
 18. The method asrecited in claim 15 wherein said alignment targets are formed accordingto a positive resist process.
 19. The method as recited in claim 15wherein said alignment targets are formed according to a negative resistprocess.
 20. The method as recited in claim 15 wherein each of saidalignment targets comprises a plurality of rectangles.